A fuel cell is an energy conversion device which converts chemical energy of fuel into electrical energy by electrochemically reacting a fuel gas and an oxidizing agent gas, and is used for industrial, domestic, and vehicle power sources, and the fuel cell may be used to supply electric power to small-sized electric/electronic products and mobile devices.
Research has been conducted on proton exchange membrane fuel cells (PEMFC, polymer electrolyte membrane fuel cells), which have high electric power density, as fuel cells for current vehicles.
FIG. 1 is a cross-sectional view schematically illustrating a basic configuration of a unit cell of a polymer electrolyte membrane fuel cell.
As illustrated, a membrane electrode assembly (MEA) 1, which is a main constituent component, is positioned at an innermost side of each unit cell of the fuel cell.
The membrane electrode assembly 1 includes a solid polymeric electrolyte membrane which may move hydrogen ions, and a cathode and an anode which are electrode layers on which catalysts are applied at both surfaces of the electrolyte membrane so that hydrogen and oxygen may react.
Gas diffusion layers (GDL) 2 are stacked at the outside of the membrane electrode assembly 1, that is, the outside where the cathode and the anode are positioned, and bipolar plates 3 and 4, which have flow paths through which reactant gases (hydrogen which is a fuel gas, and oxygen or air which is an oxidizing agent gas) are supplied and a coolant passes, are positioned at the outside of the gas diffusion layers 2.
Gaskets 8 and the like for fluid sealing are interposed and stacked between the bipolar plates 3 and 4, and the gaskets 8 may be formed integrally with the membrane electrode assembly 1 or the bipolar plates 3 and 4.
Assuming that based on the membrane electrode assembly 1 in FIG. 1, a left bipolar plate 3 is an anode bipolar plate, and a right bipolar plate 4 is a cathode bipolar plate, channels 5 between the gas diffusion layer 2 joined to the anode of the membrane electrode assembly 1 and the anode bipolar plate 3 are anode channels through which hydrogen, which is a fuel gas, flows.
Channels 6 between the gas diffusion layer 2 joined to the cathode of the membrane electrode assembly 1 and the cathode bipolar plate 4 are cathode channels through which air (oxygen), which is an oxidizing agent gas, flows, and spaces, which are formed by bipolar plate land portions 3a and 4a between the neighboring anode channels 5 and between the neighboring cathode channels 6, are coolant channels 7.
This configuration is applied as a unit cell, a plurality of cells is stacked, end plates (not illustrated), which support the cells, are coupled at outermost sides of the cells, and the end plates and the cells are fastened together by a stack fastening mechanism (not illustrated) in a state in which the cells are stacked and arranged between the end plates, thereby configuring a fuel cell stack.
Each of the unit cells maintains a low voltage in operation, and as a result, dozens or hundreds of cells are manufactured in the form of a stack by being stacked in series in order to increase voltage, and used as an electric generator.
FIG. 2 is a cross-sectional view schematically illustrating a fuel cell stack configured by stacking cells, and end plates are coupled to both ends of a stack 10, which are outer sides of cells 9, in a state in which the cells 9 are stacked.
In this case, a penetrated end plate 11, which has a manifold hole 11a, may be coupled to one end of both ends of the stack 10, and a non-penetrated end plate 12, which has no manifold hole, may be coupled to the other end opposite to the one end.
In this configuration, when hydrogen, air, and a coolant, which are supplied through inlet manifolds 13 of the stack 10 (the inlet manifolds for hydrogen, air, and a coolant are separated from each other), are distributed into the respective cells 9 through the flow paths of the bipolar plates, the hydrogen, the air, and the coolant pass through the anode channels, the cathode channels, and the coolant channels, respectively, which are flow paths of the bipolar plates in each of the cells, and thereafter, foreign substances such as unreacted gas, nitrogen, and water, and the coolant are discharged from the respective cells 9 and the stack 10 through an outlet manifold 14.
Current collecting plates 15 and 16, which have terminals 15a and 16a, respectively, are provided in the end plates 11 and 12, respectively.
The current collecting plates 15 and 16 are electrically connected to the bipolar plates of end cells positioned therein, and output electric current, which is generated by a fuel cell reaction in all of the cells 9 in the stack 10, to the outside through the terminals 15a and 16a. 
In the polymer electrolyte membrane fuel cell, water (produced water) is inevitably produced at the cathode side as a result of the reaction between the fuel gas (hydrogen in the reactant gas) supplied to the anode channel and the oxidizing agent gas (air or oxygen in the reactant gas) supplied to the cathode channel.
When the produced water remaining in the cells after stopping the operation of the fuel cell is left at a low temperature, the produced water may be frozen, and may then block pores of the gas diffusion layer (GDL) and the channels of the bipolar plate (flow paths of the reactant gas).
For this reason, the reactant gas cannot flow smoothly at the time of a cold start, and as a result, the fuel cell cannot operate normally, and starting time may be delayed.
Therefore, to ensure cold start performance and shorten starting time, it may be necessary to melt frozen moisture using electrochemical reaction heat from the fuel cell itself, and to melt frozen moisture using additional means such as a separate heating source.
FIG. 3 is a view illustrating a temperature distribution of the cells under a normal operating condition of the fuel cell stack.
Since the fuel cell stack is configured by the plurality of unit cells, the end plates having high rigidity are used in order to press and fasten the cells with uniform pressure.
The end plate is manufactured using a material such as metal having high thermal mass in order to maintain high rigidity, and as a result, as illustrated in FIG. 3, even in a case in which the fuel cell operates normally, an operating temperature of the cell at an end of the stack, which is adjacent to the end plate, is lower than that of the cells inside the stack, thereby showing lower electric power generation efficiency.
In particular, because of heat loss, the cells at the end of the stack require a longer amount of time in order to melt the frozen water at the time of a cold start, which causes a delay in the starting time.
It has been reported that voltage of the cells at both ends of the stack may be greatly lower than that of the cells inside the stack under a cold start condition of the fuel cell.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.